Determination of coordinates of points on the surface of an object is often used for digitizing or imaging of the object, or for various manufacturing applications. Some of the known coordinate-measuring probes are based on conoscopic holography.
The theory of conoscopic holography, a technique implementing interference of light (which may be spatially incoherent, unpolarized, and/or quasi-monochromatic) emanating from an object for the purposes of retrieving information about the shape of the object, has been developed by Gabriel Sirat et al. (see, e.g., JOSA A, v. 9, pp. 70-90, 1992, and references therein, all incorporated herein by reference). The use of spatially incoherent light makes it possible to use this technique in a large variety of environments. Moreover, the spatial resolution of conoscopic holography in conjunction with photodiode arrays provides for digital processing of the resulting holograms.
In the basic interference set-up, shown in FIG. 1, an object 1, illuminated with incident light, reflects the light (specularly and/or diffusely) within a solid angle A. The reflected light ri passes through a circular polarizer P1, thereby generating two beams ro and re with mutually orthogonal polarizations (in phase quadrature), both of which (and ordinary and extraordinary polarizations, respectively) propagate through a uniaxial crystal 2 having a crystal axis 3, along approximately the same geometrical path. These two rays are converted back to the same polarization mode by a following circular analyzer P2, placed after the crystal 2, and so interfere in the observation (or recording) plane 4. The circular analyzer P2 also compensates for the initial quarter-wavelength delay that the ordinary and extraordinary beams acquire upon propagation through the circular polarizer P1. The interference pattern appearing in the observation plane 4 is a conoscopic hologram and represents a superposition of the conoscopic figures for each point 5 of the object 1. The conoscopic figures for each point 5 (or for a well-defined set of points) will be referred to herein as “elementary” conoscopic figures. Each elementary conoscopic figure is formed by interference of light emanating from a particular object point, and is shaped, in part, according to the position of the emitting object point relative to the fixed recording plane 4. Each point of the object creates its own conoscopic figure, which reveals the transverse position of the point (based on position with respect to the center of the pattern) and distance (based on the density of interferometric fringes). Thus, the conoscopic hologram contains complete information about distances between the emanating object points and the recording plane, and, therefore the object's spatial distribution.
Conoscopic holography, linear or quadratic, may be utilized in many applications such as quality control measurements, digitizing, reverse engineering and in-process inspection. Several methods of optical or numerical reconstruction of conoscopic holograms, allowing for the retrieval of information about the shape of an illuminated object, and the description of corresponding systems have been reported to-date. For example, laser sensors ConoProbe™ and ConoLine™, developed by Optical Metrology Ltd. (Optimet) of Jerusalem, Israel (http://optimet.com/optimet_company_profile.htm) on the basis of conoscopic holography, provide contactless three-dimensional measuring of surfaces with submicron resolution. Conoscopic holography is the subject of various patents, including U.S. Pat. Nos. 4,602,844, 4,976,504, 5,081,540, 5,081,541, and 7,375,827, each of which is incorporated herein by reference. In particular, linear conoscopic holography and systems have been disclosed in U.S. Pat. No. 5,953,137, which is also incorporated herein by reference.
In applications such as dental surface profiling for purposes of reconstruction, orthodontics etc., the relative movement of the patient's mouth with respect to the sensor and other vibrations during the tooth-measurement cycle would impose practical limitations on performance of the existing systems which are not configured for operation within a human mouth. Clearly, an automated and robust solution to the problem of quick digitizing of complex bodies is desirable. It was also recognized in prior art that performing surface and distance measurements on translucent objects such as teeth with conventional techniques such as a three-dimensional automated scanning (see, e.g., WO 2007/071306 to Durbin et al.) results in projected images that are blurred because of the diffusion of light throughout the object. To overcome such limitation, prior art scanners employ opacifying the area of a scene to be imaged by applying an appropriate coating to it.